There has conventionally been known an operating machine that has a supporting body, a slewing body supported turnably on the supporting body, a boom attached so as to be raised and lowered with respect to the slewing body, a slewing motor for slewing the slewing body, a boom cylinder for raising and lowering the boom, a hydraulic pump for supplying hydraulic fluid to the slewing motor and boom cylinder, a flow rate control valve for controlling the supply and discharge of the hydraulic fluid to and from the slewing motor and the boom cylinder, and a throttling valve provided in a meter-out oil passage extending from the slewing motor and the boom cylinder.
This type of operating machine controls the actuation of the slewing motor and the boom cylinder by adjusting the flow rate of the hydraulic fluid that flows from the hydraulic pump and operating the flow rate control valve. When, for example, lowering the boom, the potential energy corresponding to the level of the boom before the lowering acts in a direction in which the boom is accelerated. This potential energy is discarded as thermal energy that is generated when the hydraulic fluid passes through the throttling valve. Similarly, when decelerating the slewing motion of the slewing body, inertial energy of the slewing body acts in a direction interfering with deceleration of the slewing body. This inertial energy, too, is discarded as thermal energy that is generated when the hydraulic fluid passes through the throttling valve.
A hydraulic control device disclosed in Patent Document 1, for example, is known as the technology for regenerating these energies. The hydraulic control device disclosed in Patent Document 1 has an engine, a hydraulic pump having a drive shaft coupled to a rotation axis of the engine, a variable capacity-type hydraulic motor having a drive shaft coupled to the drive shaft of the hydraulic pump, an actuator activated by the supply of hydraulic fluid from the hydraulic pump, a switching valve for controlling the supply and discharge of the hydraulic fluid to and from the actuator, a pilot pump that generates pilot pressure for operating the switching valve. The hydraulic control device disclosed in Patent Document 1 rotates the engine by supplying the hydraulic fluid, which returns from the actuator, to the variable capacity-type hydraulic motor. Thereby regeneration of hydraulic energy can be accomplished.
In the hydraulic control device disclosed in Patent Document 1, the variable capacity-type hydraulic motor is constantly rotated by the engine even when the hydraulic energy regeneration is not accomplished. In such a case, for the purpose of suppressing the occurrence of cavitation in the variable capacity-type hydraulic motor, the hydraulic fluid is fed from the pilot pump to the variable capacity-type hydraulic motor at all times.
In the hydraulic control device disclosed in Patent Document 1, the variable capacity-type hydraulic motor is rotated by using some of the hydraulic fluid supplied from the pilot pump to the switching valve, which, in other words, some of the power for operating the switching valve. This results in a loss of power of the pilot pump in an effort to prevent the occurrence of cavitation in the variable capacity-type hydraulic motor.
The hydraulic control device disclosed in Patent Document 1 also has a check valve for preventing the hydraulic fluid, which serves to the energy regeneration, from being introduced to a pilot circuit. Specifically, this check valve allows the hydraulic fluid to flow from the pilot pump to the variable capacity-type hydraulic motor, and at the same time restricts the hydraulic fluid from flowing from the variable capacity-type hydraulic motor to the pilot pump. The discharge pressure of the pilot pump is set high enough to operate the switching valve. Therefore, the cracking pressure for opening the check valve also needs to be set at a relatively high level. For this reason, in the hydraulic control device disclosed in Patent Document 1, a significant amount of power that is calculated by multiplying the cracking pressure by a supply flow rate of the hydraulic fluid supplied to the variable capacity-type motor is lost.
Patent Document 1: Japanese Unexamined Patent Publication No. 2003-120616